I was interested to read Bill Bradley's issues with the cruise control in his manual 2008 Subaru Impreza, as I encountered exactly the same problem with mine. The dealer told me that it was a common problem with this model and that due to it being a serious safety issue the car had been recalled. It was a computer problem and took half an hour for the dealer to fix free of charge and my Subaru hasn't missed a beat since. I would strongly urge Bill to contact Subaru.
It doesn't appear to be a safety recall as it doesn't show up on the government recall list, but it could be a Subaru service alert, in which case owners with this issue should approach their dealer and request it be fixed.